1. Field of the Invention
The invention is directed to an ultrasound apparatus having a first group of transmission and reception channels, each of which is allocated to an elementary transducer in the active aperture of a connectable transducer array, wherein the central transmission location generated by a transmission channel and the central reception location sampled by a reception channel coincide and lie on the allocated elementary transducer.
2. Description of the Prior Art
U.S. Pat. No. 4,409,982 discloses an ultrasound apparatus having transmission and reception channels that are each allocated to an elementary transducer in the active aperture of a connectable transducer array. The respective central transmission and reception locations coincide and lie on the allocated elementary transducer. Each transducer is connectable via an analog multiplexer to a transmission or reception channel. The central transmission location is identical to the origin of the beam direction of the primary energy of an elementary transducer, whereas the central reception location is identical to the origin of the primary direction of sensitivity of an elementary transducer. The array division, i.e. the spacing of the elementary transducers on the transducer array, thus prescribes the spacing of the central transmission and reception locations.
Simulations and measurements at commercially available transducer arrays, particularly at "curved arrays" having a surface convexly curved in the emission direction, have shown that the dynamics of the dot image function is limited to approximately 40 dB through 50 dB. This is predominantly due to the relatively coarse divison of the array, for example a grid width of 0.75 mm (corresponding to the spacing of the central transmission and reception locations) at 3.5 MHz and given a radius of curvature of 70 mm. This dynamics limitation places a limit on the capability of ultrasound apparatus to recognize low contrast differences. It is desirable, however, to make the array division as fine as possible. This is because the more finely divided the transducer array, the better an electronic sweep or an electronic focusing can be implemented. A fine array division is the basis of an image presentation with high resolution. For reasons of outlay given extremely high-frequency arrays (7.5 MHz) and technological limits, because elementary transducers become extremely narrow, the number of elementary transducers cannot be arbitrarily increased in array systems. The result is that disturbances called grating lobes appear. Grating lobes arise due to the finitely fine, spatial quantization by the array division or, in other interpretations, due to diffraction at the grating grid of the array division.